Tin-containing ilmenite structure compounds (e.g. ZnSnO3) have attracted attention for use in various applications including gas sensors, moisture sensors, electrodes for solar cells, and transparent conductive materials. Tin-containing ilmenite structure compounds such as ZnSnO3 are, however, difficult to be generated. In addition, it is reported that, at temperatures exceeding 700° C., a reaction represented by 2ZnSO3→Zn2SnO4+SnO2 occurs, preventing stable existence of a tin-containing ilmenite structure compound (Non-Patent Documents 1 to 4). Under such circumstances, a tin-containing ilmenite structure compound having thermal stability has been desired.
It is also reported that a tin-containing ilmenite structure compound such as ZnSnO3 has a resistance lower than that of a spinel structure compound such as Zn2SnO3 (Non-Patent Document 5). However, since a tin-containing ilmenite structure compound is unstable at temperatures exceeding 700° C., no studies have been made on the use of a tin-containing ilmenite structure compound in a sintered target which is normally produced by treating at high temperatures. Also, no studies have been made on a transparent conductive film formed using such a target by sputtering, pulse laser deposition or ion-plating.
Further, in applications of gas sensors, moisture sensors, electrodes for solar cells, no studies have been made on a tin-containing ilmenite structure compound containing indium in an amount exceeding a certain level.
In recent years, development of displays has been remarkable. A liquid crystal display (LCD), an electroluminescence display (EL), a field emission display (FED), or the like is used as a display device for business machines such as personal computers and word processors, and display devices for control systems in factories. These displays have a sandwich structure in which a display device is held between transparent conductive oxides.
A main stream material for such a transparent conductive oxide is indium tin oxide (hereinafter occasionally abbreviated as “ITO”) prepared by a sputtering method, an ion plating method, or a vapor deposition method as described in Non-Patent Document 1.
ITO is composed of a specific amount of indium oxide and tin oxide, possesses excellent transparency and conductivity, can be etched using a strong acid, and exhibits high adhesion to a substrate.
Although ITO (a composite oxide of indium and tin which normally contains tin in an amount of about 5 to 15 atomic percent) has excellent properties as a material for transparent conductive oxide, ITO is not only a scarce resource, but also contains a large amount (about 90 atomic percent) of indium which is a biologically harmful element. Moreover, the indium itself produces nodules (projections) during sputtering. The nodules produced on the target surface have been one of the causes of abnormal electrical discharge. In particular, when an amorphous ITO film is produced for improving etching properties, the indium compound on the surface of the target is reduced due to introduction of a small amount of water and hydrogen gas in the sputtering chamber, giving rise to further production of nodules. If an abnormal electrical discharge occurs, scattered materials become attached to the transparent conductive oxide as impurities during or immediately after the film formation.
The indium content in ITO must be reduced due to these problems of instability of supply (scarcity), hazardous properties and nodule generation during sputtering. If an attempt is made to reduce the indium content of ITO to 80 atomic percent or less, a high-resistance tin compound in a target has electric carriers (charges), leading to problems such as a higher likelihood in the occurrence of abnormal electrical discharge and difficulty in etching with an acidic aqueous solution.
As a method for preventing generation of nodules and suppressing abnormal electrical discharge, incorporation of a hexagonal layered compound of In2O3(ZnO)m, wherein m is an integer of 2 to 20, with a crystal grain diameter of 5 μm or less has been investigated (Patent Documents 1 and 2). However, if the indium content is reduced to 70 atomic percent or less in this method, there are problems such as decrease of the sintered density and conductivity of the target, which causes abnormal electrical discharge and retards the film forming speed; low target strength, leading to easy cracking; and poor heat resistance in the presence of air of the transparent conductive film formed by sputtering.    Patent Document 1: WO01/038599    Patent Document 2: JP-A-06-234565    Non-Patent Document 1: Solid State Ionics Volume 109, Issues 3-4, 2 Jun. 1998, pp. 327-332    Non-Patent Document 2: Z. Anorg. Allg. Chem. 527 (1985), p. 193    Non-Patent Document 3: Z. Anorg. Allg. Chem. 527 (1985), pp. 193-202    Non-Patent Document 4: Kh. S. Valeev, E. I. Medvedovskaya, S. D. Notkina, T. Gosudarst, Issledovatel. Elektrokeram. Inst. 4 (1960) 80 (in Russian)    Non-Patent Document 5: “Technology of Transparent Conductive Film” edited by The 166th Committee of Transparent Oxide and Photoelectron Material, Japan Society for Promotion of Science, Ohmsha, Ltd. (1999)
An object of the invention is to provide a material having a tin-containing ilmenite structure compound, a method for producing thereof, a low-resistance, high-strength target with a high theoretical relative density using the material, a target having a small indium content, a target which allows sputtering to be performed stably while suppressing abnormal electrical discharge which occurs during formation of a transparent conductive film by sputtering, a transparent conductive film, formed using the targets, which has a low in-plane distribution of specific resistance and a transparent electrode.